This invention relates to a digital signal transmitting method for performing the transmission of data by time division multiplex by using a communicating cable.
With the increase in the types and uses of computers and the development of digital signal processing techniques, a significant amount of attention has been directed to data communication of the type wherein communicating systems and data processing systems are combined to perform on-line information processing. In particular, for small scale communication systems, such as would be used for a single company office or a local government having multiple offices, a local area network system which uses a packet form of communication and which communicates via cable, such as a coaxial cable, has attracted a lot of attention because of its economy, reliability and high transmitting efficiency.
In a packet type communicating system, the communicating cable for performing bidirectional transmission of information is installed at a laboratory or the like, and the multiple personal stations are connected thereto. The transmission of messages, divided into data blocks of 1000-2000 bits, for example, is performed from the respective personal stations. The address and the other overhead bits such as serial number, are added to the message. In such a communicating system, the network itself does not have any controlling function but is a passive medium, and the control function is completely dispersed to the respective personal stations. Accordingly, the personal stations start transmitting their respective messages in packet form after they confirm the vacancy of the transmitting path. When a collision occurs between packets from two stations, both stations stop their transmission. The stations which stopped the transmission attempt to retransmit their messages after waiting a random amount of time.
In such a communication system, not only do the users at the respective stations have access to a central computer, but also they can use the hardware and software resources located at the other stations. That is to say, in this communicating system, the units which have been centralized to a central large-sized computer in a time sharing system (TSS), such as a high-speed and/or; printer high quality, a large volume file, can be used in the condition as if they were dispersed to the respective stations. Accordingly, not only is there a saving of resources and an improvement of efficiency, but also the development of large software systems by the mutual utilization of program and data is made possible.
In a communicating system of the type described, the order of priority among the personal stations for using the communicating path is not given, but all stations have equal right of access to the communicating path. Accordingly, master or slave classes, which are often found among the stations in other communicating systems, are not found in the equal access type systems, and communication between the connected arbitrary stations is made possible. Further, the transmitting path, such as a coaxial cable, is completely composed of passive circuitry, so that a highly reliable system can be easily formed.
As mentioned above, although the communicating system has various merits, the collision of packets of information on the same transmitting path possibly occurs because the respective personal stations arbitrarily start their transmission of data. The collision of packets increases as the frequency of use of the transmitting path increases.
In order to resolve the problem, a signal transmitting method known as Priority Ethernet or Reservation Ethernet has been proposed. In the Priority Ethernet method, the order of priority of the transmission of signals by the personal stations is assigned to a preamble in the packet. If a collision of packets occurs, the packet of higher priority is transmitted prior to the other one. In the Reservation Ethernet method, a master station for designating modes is assigned at all times. Thereby, whether or not there is a signal to be transmitted by each of the personal stations and the quantity of information to be transmitted are confirmed in a reservation mode. And according to the result, the order of packets to be transmitted by the personal stations are decided in each frame and the transmission of signal is time divisionally performed in the transmission mode.
However, according to the proposed former signal transmitting method, the problem of the scattering in the transmission delay time caused by the collision of the packets of the same priority, has not yet been resolved. Accordingly, the method is unsuitable for real time transmission which attaches importance to the corresponding relation of transmission and reception in real time, such as conversation-type aural communication. Further, according to the latter signal transmitting method, the above-mentioned equality among the personal stations is lost by the existence of the master station. That is, according to this method, if same failure occurs at the master stations, the data transmission from any station can not be performed, and the reliability of the entire system is degraded in this sense.